Passive automatic antifoam delivery system for use with single-use bioreactors

ABSTRACT

The aspects of the disclosed embodiments generally relate to an apparatus which allows for the controlled addition of antifoam to the foam present in the headspace of a disposable single-use bioreactor in a reliable manner. The aspects of the disclosed embodiments also generally relate to a method of using such apparatus which allows for the controlled addition of antifoam to the foam present in the headspace of a disposable single-use bioreactor in a reliable manner. The aspects of the disclosed embodiments generally relate to antifoam systems, methods and apparatus, and more particularly, to an antifoam device operably connected to a single use biobag.

FIELD

The aspects of the disclosed embodiments generally related to anapparatus which allows for the controlled addition of antifoam to thefoam present in the headspace of a disposable single-use bioreactor in areliable manner. The aspects of the disclosed embodiments generallyrelated to also directed to a method of using such apparatus whichallows for the controlled addition of antifoam to the foam present inthe headspace of a disposable single-use bioreactor in a reliablemanner. The aspects of the disclosed embodiments generally relate toantifoam systems, methods and apparatus, and more particularly, to anantifoam device operably connected to a single use biobag.

BACKGROUND

A bioreactor is a device or apparatus in the form of a closed chamber orvessel in which living organisms such as mammalian cells, bacteria oryeast synthesize substances useful to the pharmaceutical and biotechindustries under controlled conditions favorable to that specificorganism. Traditionally bioreactors were closed, rigid stainless steelvessels in which the organisms were grown. A relatively recentdevelopment has been the appearance of systems specifically designed touse disposable, single-use flexible liners or bags to provide thesterile envelope for the cells which is supported by a rigid externalsupport structure. Single-use when used in the context of a single-usebioreactor is generally acknowledged to mean a flexible container, lineror bag incorporating all of the functional aspects required of atraditional bioreactor which can be filled with the materials requiredfor the growth of mammalian cells, bacteria or yeast and is designedwith the intention that it be disposed of at the completion of a singleproduction run. Disposable in this instance means that the device isdesigned to be low cost and to incorporate materials which can be easilydisposed of using commonly available waste processing infrastructure andnot require special disposal requirements. The advantages of disposablesingle-use systems are: the elimination of the complicated productionplant infrastructure piping and systems required to clean and sterilizea rigid vessel in place, the elimination of the system downtime requiredfor the cleaning process, elimination of the materials, time and effortrequired to validate the sterility of the cleaned vessel, eliminationthe handling requirements for the caustic chemicals used in the cleaningprocess and the elimination of wastes that are generated as part of thetraditional cleaning process. The disposable, single use liner or bag isdelivered to the customer as a closed, sterilized container which can beeasily disposed of when the production run has been completed. Theturnaround time for a single-use bioreactor system is greatly reducedsince it basically consists of the a quick sanitization of the used bag,removal of the used bag from the support structure/vessel and theinstallation of the new bag to be used for the next production run intothe support structure/vessel. U.S. Pat. No. 7,629,167 issued Dec. 8,2009 describes many such bioreactors.

One of the main disadvantages of single use, disposable bioreactors isthat the plastic films used to create the flexible liners or bags arenot high strength materials. The plastic films need to be thin so thatthey are flexible enough to be easily handled during installation intothe external rigid support structure. Using thin plastic films alsoallow the single use bioreactor to be able to be folded into a smallersize package for shipment to customers. When microorganisms are growninside a bioreactor their metabolism generates heat which must beremoved from the bioreactor to prevent it from building up to levelsthat can inhibit growth of the microorganisms or even result in thedeath of the microorganisms. The heat produced by the microorganismsinside the bioreactor must pass through the flexible bioreactorcontainer wall in order for it to be removed from the system. Plasticmaterials are not good conductors of heat so the thinner the plasticfilm that comprises the walls of the disposable, single-use flexiblebioreactor bags can be, the better the rate of heat transfer that can beachieved. For these reasons the thin plastic films used to make theflexible bioreactor bag results in a bag or container that cannotwithstand high pressures. Pressures of just a few pounds per square inchcan create leaks in the envelope of these bags and this compromises theintegrity of the bag and renders such a compromised bag susceptible tocontamination by harmful organisms in the environment external to thebag.

Growing mammalian cells, bacteria or yeast in a bioreactor also oftenresults in the production of an unwanted foam layer which floats at thetop of the liquid in the bioreactor. This foam layer is the result ofseveral factors. Microorganisms commonly used in the production ofuseful pharmaceutical or biological substances are aerobic, that is theyrequire air to survive. In a standard stirred vessel bioreactor therequired pressurized air is introduced at the bottom of the vessel inthe form of small air bubbles and the vessel contains one or moreimpellers which are used to mix or stir the air bubbles into the liquidand to break the bubbles into smaller bubbles if possible. Even thoughthey are being agitated or mixed by the action of the impeller(s) thebuoyancy of these bubbles causes them to eventually rise to the top ofthe liquid surface. The liquid growth media used to grow livingorganisms in a bioreactor contains a wide array of substances andmaterials which are used as basic nutrients and growth factors by eachspecific type of organism that might be grown in a bioreactor. Many ofthese materials required by the organisms to survive also promote theformation more stable bubbles and thus a more stable foam layer at thesurface of the liquid than would be the case for bubbles formed in purewater. Living organisms in a bioreactor also generate waste products aspart of their metabolism and some of the waste materials also contributeto the formation of stable bubbles and foam. A constant flow ofpressurized air is required to be introduced into the bottom of thebioreactor and this produces the constant creation of more bubbles andfoam to be added to the existing foam layer. Bubbles making up the foamlayer at the top surface of the liquid will have some lifetime duringwhich they will persist, but eventually they will burst. If the foamlayer has been made more stable as described in the preceding text, thenthe thickness of the foam layer will increase until the rate of bubbleformation and the rate of bubbles bursting reaches equilibrium. Thethickness of this foam layer in the absence of any antifoam compound maybecome unacceptably thick. To reduce this foam layer at the top of abioreactor to a reasonable thickness various commercially availableantifoam compounds have been developed. The effectiveness of theseantifoam compounds is not constant over time requiring that severalapplications of the antifoam compound be applied to the foam during asingle production run. The rate of foam production and the effectivenessof the antifoam in reducing the form layer produced varies widelydepending on the particular materials and conditions required tocultivate each different type of microorganism in a bioreactor.

There is a space at the top of the single-use bioreactor bag called aheadspace which is intended to be used to as a space for some amount ofthis foam to exist in. The intent is for this headspace to be largeenough to contain a reasonable foam layer thickness and also to be largeenough to include an additional clear space above the top of the foamlayer. At the top of the bioreactor bag above where the top of the foamlayer is expected to end is a ported opening in the bag wall to which atube external to the bag is connected which provides a closed pathway toan exhaust filter through which the exhaust air flow can exit the bag.The exhaust filter allows waste gasses to flow out of the bag butprevents potentially harmful organisms in the environment external tothe bag from entering the inside of the bag where they could contaminatethe desired population of organisms. The exhaust tubing and the exhaustfilter during normal operating conditions will allow the constant flowof exhaust gasses through this pathway without creating an undesirablelevel of back pressure. If the foam layer at the top of the liquid inthe headspace of the bag becomes thicker than the headspace canaccommodate then some of the foam can be drawn into the exhaust tubeand/or exhaust filter by the flow of the exiting exhaust gas. This foamcan decrease the effective size of the flow path though the exhausttubing or can be deposited on the exhaust filter porous filter materialreducing its effective surface area both of which will restrict theexhaust gas flow rate out of the bioreactor bag. A restriction in theexhaust gas flow rate from the bioreactor bag will increase the backpressure in this exhaust path and thus increase the pressure within thebioreactor bag itself. Since single-use bioreactor bags are made of thinflexible sheets of plastic film they are inherently low pressuresystems. A high enough back pressure in the exhaust gas pathway canincrease the pressure within the bioreactor bag to levels which maycompromise the integrity of the bioreactor bag.

In an effort to control the level of foam at the top surface of theliquid in a bioreactor different antifoam addition strategies have beendeveloped. For a few cultures of microorganisms in which the materialsrequired for growth do not promote stable foam or where required airflow rates are low a single application of a small amount of antifoam atthe start of the production run may be sufficient to control foamlevels. Other microorganisms may require materials in the growth mediaor air flow rates which require antifoam to be applied several timesduring a production run, possibly on some predefined schedule. Mostmicroorganisms require materials in the growth media or air flow atrates that lead to excessive foam generation and thus call for moreelaborate strategies of foam control. One of the simplest strategies ofantifoam addition depends on human operators to observe foam levelsduring a production run and add antifoam when they determine that it isneeded. This strategy is inherently subjective and may lead to theaddition of too much antifoam which would need to be removed bydownstream purification processes or the addition of too little antifoamwhich risks blocking the exhaust gas path. Depending on the requirementsof the downstream purification process, after the conclusion of aproduction run any residual antifoam present in a culture may need to beremoved from the process fluid stream during downstream processing. Theantifoam itself may have minor detrimental effects on the growth rate ofthe microorganism during the production run. These factors cause anyantifoam addition strategy to be based on adding the absolute minimum ofantifoam required to reliably control the foam level.

In theory an automated system for antifoam addition would only addantifoam as needed. The basic functional blocks of such an activecontrol system would be a sensor, a system controller, and an actuator.The overall foam control system would be connected such that there is afeedback pathway for foam level information from the foam sensor to beinput to the system controller and output pathway for a control signalto be sent from the system controller to the actuator. The foam sensormeasures some physical aspect of the foam and sends that information tothe system controller. The system controller can take that sensorinformation and determine if it needs to act on that information. If thesystem controller determines that action is required it can send acontrol signal to the actuator that causes the actuator to perform someaction that initiates the addition of antifoam which in turn affects thefoam level in a manner that is desirable.

An antifoam control system for a single use bioreactor can be describedin more detail as follows. An active control system for bioreactor foamcontrol requires a sensor that can measure some physical aspect of foamsuch that it can determine if foam is present in the headspace of thebioreactor and ideally correlate this physical measurement to the amountof foam present. For example the foam sensor can be based on somephysical electrical measurement such as the resistance or conductance atsome location in the bioreactor headspace. Another example of a foamsensor is based on some optical property measured at some location inthe bioreactor headspace that performs the same function.

An active control system for bioreactor foam control requires a systemcontroller that can take the measurement signal from the foam sensor anddetermine if it needs to act on that information. An example of a systemcontroller for foam control could be a digital controller such as amicrocontroller or a microprocessor. Another example of a systemcontroller for foam control could be an analog controller based on ananalog comparator. Another example of a system controller for foamcontrol could be a human who takes the information from the sensor anddetermines if action should be taken to reduce the foam level

An active control system for bioreactor foam control requires anactuator that can be controlled by the system controller to perform someaction that affects the level of foam present in the bioreactorheadspace as desired. An example of an actuator used in a foam controlsystem could be a valve that allows chemical antifoam to be added to thebioreactor headspace which would reduce foam. Another example of anactuator used in a foam control system could be a pump that allowschemical antifoam to be pumped into the bioreactor headspace to reducefoam. Another example of an actuator used in a foam control system couldbe a mechanical agitator such as an impeller located in the bioreactorheadspace which mechanically breaks up the foam. Another example of anactuator used in a foam control system could be a human who manuallyadds chemical antifoam to the bioreactor headspace or performs someaction that reduces the level of foam as desired.

Such active foam control systems are presently available for foamcontrol in bioreactors but are not widely used. The reliability of foamcontrol systems at the present time is not high enough to make automatedsystems widely accepted. Problems inherent in such active controlssystems are incorrect information from the foam sensor, inappropriateaction taken by the system controller and ineffective effects by theactuator on the foam level.

Incorrect information from the foam sensor can take the form of nodetection of the existing foam level (false negative), information thatdoes not correlate to the foam level or, an incorrect indication thatfoam is present (false positive). These errors in detecting foam by foamsensors can arise from the difficulties inherent in detecting foam dueto differences in the measurable properties of foam in differentbioreactor process conditions. For example different bioreactor processconditions can generate foam with different electrical properties anddifferent optical properties. Condensation of moist air in the headspaceof a bioreactor into droplets on the foam sensor detecting element maybe falsely interpreted by the sensor as being foam. Errors in detectingfoam by foam sensors can also be due to problems related to thatspecific instance of foam sensor, i.e. that particular foam sensor hasfailed. The reliability of foam sensors at the present time is not highenough to make automated foam control systems widely accepted.

Since the system controller is the core of a foam control feedbacksystem inappropriate action taken by the system controller can be due tonot tailoring the system controller's control strategy to differences inbioreactor process conditions. This inappropriate action can be causedby incorrect inputs to the system controller or by incorrect levels ofoutput by the controller to the actuator. An example would be whendifferent bioreactor process conditions generate foam with differentphysical properties which affect the accuracy of the foam level readingfrom the foam sensor. Another example would be that different bioreactorprocess conditions can generate foam with different physical propertieswhich affect the effectiveness of the system foam control method inreducing the level of that particular foam. Errors in system controllerresponse to foam can also be due to problems related to that specificinstance of system controller, i.e. that particular system controllerhas failed.

There is thus a great need to improve foam control.

SUMMARY

The aspects of the disclosed embodiments generally related to anapparatus which allows for controlled addition of antifoam to foampresent in the headspace of a disposable single-use bioreactor in anefficient reliable manner.

The basic configuration of the apparatus consists of one or more pads orwicks made of porous or fibrous materials which are located inside theheadspace of a single-use bioreactor bag in close proximity to theexhaust gas exit port(s). The porous or fibrous wicks or rings areoperably connected to an external rigid or flexible reservoir attachedto the bag through a port fitment welded into the bioreactor bag wall.

The port fitment is connected to tubing which creates an enclosedpathway between the external reservoir and the wicks or rings locatedinternally to the bioreactor bag. Antifoam is introduced into theexternal reservoir by the user such as via a sterile syringe fitting orby tube welding on a small container of antifoam. The antifoam in thereservoir then flows through the enclosed pathway and is absorbed orwicked into the porous or fibrous material of the wicks or rings such asby a capillary transfer mechanism. Surface tension of the antifoam wouldensure that the antifoam remained suspended in the porous or fibrousmaterial of the wicks or rings until foam from the bioreactor rose tothe level where it made contact with the antifoam soaked wicks or rings.

Once the foam in the bioreactor rises to a level where it makes contactwith the wicks or rings a small amount of antifoam is transferred to thebioreactor foam mass. This automatic application of antifoam to thebioreactor foam reduces the level of the bioreactor foam in thebioreactor bag headspace. A capillary transfer mechanism automaticallyreplaces the antifoam in the wicks or rings that had been applied to thefoam mass in the bioreactor bag. This is a self-regulating antifoamcontrol system that requires no outside intervention to repeatedly cycleand operate as needed after the initial filling of the reservoir. Thisapparatus thus passively regulates the amount of antifoam applied to thebioreactor foam and applies the antifoam only as needed.

The passive system as described above would not exclude the possibilityof making an antifoam application on demand as desired by the user. Aforce could be applied to the external reservoir in such a manner thatit created an increased pressure inside the antifoam reservoir whichwould cause antifoam to be expelled from the wicks or rings where itwould make contact with the foam layer in the bioreactor headspace.

This apparatus is passive in that it does not require a sensor to detectfoam, nor an active control system to act on information from a foamsensor to apply a controlled amount of antifoam to the headspace of adisposable single-use bioreactor. This passive system also does notrequire a power source to operate which is in contrast to an activesystem. Thus this passive system does not suffer the reliabilityproblems associated with existing active control systems for bioreactorfoam control. Aspects of the disclosed embodiments also generally relateto a much needed integral control that is presently lacking inbioreactor systems.

One aspect of the exemplary embodiments is also directed to a passiveautomatic antifoam delivery system for use with single-use bioreactorscomprising:

a medical grade porous or fibrous non-reactive object secured proximallyto the exhaust port of the bioreactor (such as a cylindrical wick or aring);

wherein said porous or fibrous material absorbs/wicks antifoam from thebioreactor reservoir and

retains said antifoam therein until foam from the bioreactor rises tothe level wherein it makes contact with the antifoam absorbed/wickedporous or fibrous object which releases small quantities of antifoamsufficient to reduce foam bellow the exhaust port.

Another aspect of the exemplary embodiments is directed to a medicalgrade cylindrical wick or a ring of porous or fibrous material; whereinsaid porous or fibrous material is absorbed/wicked with antifoam andretains it therein until exposed to a mass of foam which causes therelease of small quantities of antifoam sufficient to neutralize thefoam from the mass.

Another aspect of the exemplary embodiments is directed to the use of amedical grade cylindrical wick or a ring of porous or fibrous materialto neutralize foam in a bioreactor reservoir comprising:

securing said porous or fibrous material around or adjacent to theexhaust port in the top of a single-use bioreactor bag;

absorbing/wicking antifoam onto the porous or fibrous material;

retaining antifoam absorbed/wicked porous or fibrous material thereinuntil exposed to a mass of foam which causes the release of smallquantities of antifoam sufficient to neutralize the foam from the mass.

Medical grade porous or fibrous material as used herein refers to abiocompatible material not having toxic or negative effects on thegrowth of organisms commonly used in the pharmaceutical industry or onthe useful products produced by such organisms. Such material could beformed from open cell porous foams, fibrous mesh or pads or sinteredbead foams made from a wide range of materials such as polymeric plasticmaterials like polyethylene, polypropylene, polyester, polyolefins,polyamides, polyurethane, acrylics, styrenics, etc. or from metals ormetal alloys such as titanium or stainless steel or from ceramics suchas silicon nitride, and zirconium dioxide.

Anti-foaming agents are hydrophobic agents such as polydimethylsiloxanewith silica (such as XIAMETER® products including Antifoam 2210 orCompound A AFE-1520 Antifoam Emulsion, AFE-1510 Antifoam Emulsion,AFE-0010 Antifoam Emulsion FG, ACP-1920 Powdered Antifoam, and AFE-0100AF Emulsion FG (Dow Corning), M-10 (Calgene), Breox FMT 30 (Blockcopolymer of polyethylene glycol and polypropylene glycol having amolecular weight of approximately 3,000, available from BP ChemicalsLtd.); Darastil 8231 (Block copolymer of polyethylene glycol andpolypropylene glycol having a molecular weight of approximately 2,000,available from Grace Dearborn Ltd.), Sigma-Aldrich Antifoams 204 (A6426and A8311 containing a mixture of organic non-silicone polypropylenebased polyether dispersions), A6582 (100% silicone based polymer thathas a molecular weight range of 3,200 to 16,500 Da consist of particlesranging in size from 10 to 40 microns, and can be removed byfiltration.), A6457, A6707, A8082 (completely organic, fatty acid estertype antifoam) and A8582 (Sigma Aldrich) (from about 0.001 wt. % toabout 0.005 wt. %), J673A (Struktol an alkoxylated fatty acid ester on avegetable base), P2000 (Fluka Polypropylene glycol) or SB2121(Struktol). Other commonly used antifoam agents are insoluble oils,polydimethylsiloxanes and other silicones, certain alcohols, stearatesand glycols. Oil based defoamers have an oil carrier. The oil might bemineral oil, vegetable oil, white oil or any other oil that is insolublein the foaming medium, except silicone oil. An oil based defoamer alsocontains a wax and/or hydrophobic silica to boost the performance.Typical waxes are ethylene bis stearamide (EBS), paraffin waxes, esterwaxes and fatty alcohol waxes. These products might also havesurfactants to improve emulsification and spreading in the foamingmedium. Water based defoamers are different types of oils and waxesdispersed in a water base. The oils are often white oils or vegetableoils and the waxes are long chain fatty alcohol, fatty acid soaps oresters. These are normally best as deaerators, which means they are bestat releasing entrained air. Silicone-based defoamers are polymers withsilicon backbones. These might be delivered as an oil or a water basedemulsion. The silicone compound consists of an hydrophobic silicadispersed in a silicone oil. Emulsifiers are added to ensure that thesilicone spreads fast and well in the foaming medium. The siliconecompound might also contain silicone glycols and other modified siliconefluids. EO/PO based defoamers contain polyethylene glycol andpolypropylene glycol copolymers. They are delivered as oils, watersolutions, or water based emulsions. EO/PO copolymers normally have gooddispersing properties and are often well suited when deposit problemsare an issue. Alkyl polyacrylates are suitable for use as defoamers innon-aqueous systems where air release is more important than thebreakdown of surface foam. These defoamers are often delivered in asolvent carrier like petroleum distillates.

The terms “disposable” and “single use” as used are the customary andordinary use of these terms such as found in the book is “Single-UseTechnology in Biopharmaceutical Manufacture”, Regine Eibl and DieterEibl, A John Wiley & Sons Inc.

These and other aspects and advantages of the exemplary embodiments willbecome apparent from the following detailed description considered inconjunction with the accompanying drawings. It is to be understood,however, that the drawings are designed solely for purposes ofillustration and not as a definition of the limits of the invention, forwhich reference should be made to the appended claims. Additionalaspects and advantages of the invention will be set forth in thedescription that follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. Moreover,the aspects and advantages of the invention may be realized and obtainedby means of the instrumentalities and combinations particularly pointedout in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate presently preferred embodiments ofthe present disclosure, and together with the general description givenabove and the detailed description given below, serve to explain theprinciples of the present disclosure. As shown throughout the drawings,like reference numerals designate like or corresponding parts.

FIG. 1 refers to one embodiment in which the exhaust gas exit tube isconnected to a section of the bioreactor bag wall.

FIG. 2 refers to another embodiment in which the exhaust gas exit tubeis connected to a section of the bioreactor bag wall.

FIG. 3 refers to another embodiment in which the exhaust gas exit tubeis connected to a section of the bioreactor bag wall.

FIG. 4 refers to another embodiment in which the exhaust gas exit tubeis connected to a section of the bioreactor bag wall.

FIG. 5 refers to another embodiment in which the exhaust gas exit tubeis connected to a section of the bioreactor bag wall.

FIG. 6 refers to another embodiment in which the exhaust gas exit tubeis connected to a section of the bioreactor bag wall.

DETAILED DESCRIPTION

The present disclosure is generally directed towards the use of antifoamdevices and methods so as to improve the use of bioreactor bags. As willbe understood, the various diagrams, flow charts and scenarios describedherein are only examples, and there are many other scenarios to whichthe present disclosure will apply.

Referring to FIG. 1 and FIG. 2, two embodiments of the passive antifoamsystem are illustrated. In these Figures the exhaust gas exit tube (1)is shown connected to a section of the bioreactor bag wall (4) via aport fitment (6) which is heat welded to the bag film (4). The tube canbe made of one of many materials commonly used in the pharmaceuticalindustry such as platinum cured silicone or C-Flex. The tube can beflexible, semi-rigid or rigid. There can be one or more exhaust gas exittubes attached to a bioreactor bag. The other end of each exhaust tubecan be connected to a condenser (not shown), an exhaust filter (notshown) or to another bag (not shown). Inside the bioreactor bagheadspace located in proximity to the exhaust gas port fitment is shownthe porous or fibrous material pad or wick (3) which retains theantifoam. The porous or fibrous material pad or wick (3) can be formedinto many different shapes such as the shape of a disc or ring. Theantifoam reservoir in these Figures is depicted as a tubular orcylindrical shaped container (2) which is connected to a section of thebioreactor bag wall (4) via a port fitment (6) heat welded to the bagfilm (4). An aseptic connector (5) is shown at the top of the antifoamreservoir through which the user can fill the reservoir with antifoam.This aseptic connector (5) could be replaced by a simple plug (notshown) if tube welding a sterile bag of antifoam is to be the method ofadding antifoam to the reservoir.

FIG. 3 and FIG. 4 illustrate other embodiments of the passive antifoamsystem. In these Figures, the exhaust gas exit tube (1) is shownconnected to a section of the bioreactor bag wall (4) via a port fitment(6) which is heat welded to the bag film (4). The tube can be made ofone of many materials commonly used in the pharmaceutical industry suchas platinum cured silicone or C-Flex. The tube can be flexible,semi-rigid or rigid. There can be one or more exhaust gas exit tubesattached to a bioreactor bag. The other end of the exhaust tube can beconnected to a condenser (not shown), an exhaust filter (not shown) orto another bag (not shown). Inside the bioreactor bag headspace locatedin proximity to the exhaust gas port fitment are one or more porous orfibrous material pads or wicks (3) which retain the antifoam. The porousor fibrous material pads or wicks (3) can be formed into many differentshapes such as the shape of a cylinder or tube. The antifoam reservoirsin these Figures are depicted as a tubular or cylindrical shapedcontainer (2) which is connected to a section of the bioreactor bag wall(4) via a port fitment (6) heat welded to the bag film (4). An asepticconnector (5) is shown at the top of one of the antifoam reservoirsthrough which the user can fill the reservoir with antifoam. The asepticconnector (5) could be replaced by a simple tube plug (8) if tubewelding a sterile bag of antifoam is to be the method of adding antifoamto the reservoir.

FIG. 5 and FIG. 6 illustrate other embodiments of the passive antifoamsystem. In these Figures, the exhaust gas exit tube (1) is shownconnected to a section of the bioreactor bag wall (4) via a port fitment(6) which is heat welded to the bag film (4). The tube can be made ofone of many materials commonly used in the pharmaceutical industry suchas platinum cured silicone or C-Flex. The tube can be flexible,semi-rigid or rigid. There can be one or more exhaust gas exit tubesattached to a bioreactor bag. The other end of each exhaust tube can beconnected to a condenser (not shown), an exhaust filter (not shown) orto another bag (not shown). Inside the bioreactor bag headspace locatedin proximity to the exhaust gas port fitment are one or more porous orfibrous material pads or wicks (3) which retain the antifoam. The porousor fibrous material pads or wicks (3) can be formed into many differentshapes such as the shape of a disc or ring. The antifoam reservoir inthis figure shown to be of a bag shaped container (2) which is connectedthrough a section of tubing (7) to a section of the bioreactor bag wall(4) via a port fitment (6) heat welded to the bag film (4). An asepticconnector (5) is shown at the top of a section of tubing (7) throughwhich the user can fill the reservoir with antifoam. The asepticconnector (5) could be replaced by a simple tube plug (not shown) iftube welding a sterile bag of antifoam is to be the method of addingantifoam to the reservoir.

Thus, while there have been shown, described and pointed out,fundamental novel features of the invention as applied to the exemplaryembodiments thereof, it will be understood that various omissions andsubstitutions and changes in the form and details of devices and methodsillustrated, and in their operation, may be made by those skilled in theart without departing from the spirit or scope of the invention.Moreover, it is expressly intended that all combinations of thoseelements and/or method steps, which perform substantially the samefunction in substantially the same way to achieve the same results, arewithin the scope of the invention. Moreover, it should be recognizedthat structures and/or elements and/or method steps shown and/ordescribed in connection with any disclosed form or embodiment of theinvention may be incorporated in any other disclosed or described orsuggested form or embodiment as a general matter of design choice. It isthe intention, therefore, to be limited only as indicated by the scopeof the claims appended hereto.

What is claimed is:
 1. A passive automatic antifoam delivery system foruse with a single-use bioreactor comprising: a medical grade porousnon-reactive object secured proximally to the exhaust port of thebioreactor; wherein said porous object absorbs an antifoam substancefrom a bioreactor reservoir connected to the bioreactor and the porousobject, and wherein said porous object retains said antifoam substancetherein until foam from the bioreactor rises to the level wherein itmakes contact with the antifoam absorbed porous object which releasessmall quantities of an antifoam substance sufficient to neutralize thefoam and clear the exhaust port.
 2. A passive automatic antifoamdelivery system according to claim 1 wherein said medical grade porousnon-reactive object is in the shape of a cylindrical wick or a ring. 3.A passive automatic antifoam delivery system according to claim 1wherein said medical grade porous non-reactive object is in directcontact with a small reservoir attached to the bag or tubing and theantifoam is introduced into the reservoir by a user via a sterilesyringe fitting or by tube welding of a small container of antifoam. 4.A passive automatic antifoam delivery system according to claim 1wherein said antifoam is selected from polydimethylsiloxane, blockcopolymer of polyethylene glycol and polypropylene glycol, polypropylenebased polyether dispersions, fatty acid esters, insoluble oils,polydimethylsiloxanes, mineral oil, vegetable oil, and EO/PO baseddefoamers contain polyethylene glycol and polypropylene glycolcopolymers.
 5. A passive automatic antifoam delivery system according toclaim 1 wherein said medical grade porous non-reactive object comprisesopen cell porous foams, fibrous mesh or pads or sintered bead foams. 6.A passive automatic antifoam delivery system according to claim 5wherein said medical grade porous non-reactive object comprisespolymeric plastic materials, metals or metal alloys or ceramics.
 7. Apassive automatic antifoam delivery system according to claim 6 whereinsaid medical grade porous non-reactive object comprises polymericplastic materials selected from polyethylene, polypropylene, polyester,polyolefins, polyamides, polyurethane, acrylics, and styrenics.
 8. Apassive automatic antifoam delivery system according to claim 6 whereinsaid medical grade porous non-reactive object comprises metals or metalalloys selected from titanium and stainless steel.
 9. A passiveautomatic antifoam delivery system according to claim 6 wherein saidmedical grade porous non-reactive object comprises ceramics selectedfrom silicon nitride and zirconium dioxide.
 10. A medical grade porousobject in the shape of a cylindrical wick or ring; wherein said porousobject is absorbed/wicked with antifoam and retains said antifoamtherein until exposed to a mass of foam which causes the release ofsmall quantities of antifoam sufficient to neutralize the foam from themass.
 11. A method of controlling foam formation in a single usebioreactor bag comprising: securing a medical grade porous object aroundor adjacent to the exhaust port in the top of a single-use bioreactorbag; absorbing/wicking antifoam onto the porous object; retainingantifoam absorbed/wicked porous object therein until exposed to a massof foam, and neutralizing foam in a bioreactor reservoir by the releaseof small quantities of antifoam.
 12. The use of a medical grade porousobject in the shape of a cylindrical wick or ring to neutralize foam ina bioreactor reservoir comprising: securing a medical grade porousobject around or adjacent to the exhaust port in the top of a single-usebioreactor bag; absorbing/wicking antifoam onto the porous object;retaining antifoam absorbed/wicked porous object therein until exposedto a mass of foam, and neutralizing foam in a bioreactor reservoir bythe release of small quantities of antifoam.